The present invention relates to an inkjet print head, a method of fabricating an inkjet print head, a print cartridge including such a print head and a printer arranged to operate such a print cartridge. Particularly, but not exclusively, the invention provides an inkjet print head which includes a laser source for generating acoustic waves which in turn produce a driving pressure for ink expulsion.
In the present specification, the term acoustic is used to describe a longitudinally propagating pressure wave through any of a solid, liquid or gas. In view of the operating frequency of print heads, this pressure wave may in fact have an ultrasonic frequency.
Currently inkjet print head cartridges deliver ink using two basic mechanisms, thermal or piezoelectric. Thermal systems rely on rapid heating to generate bubbles in an ink firing chamber, which expand and expel ink. Piezoelectric systems rely on the flexing of a crystal to generate a pressure wave to drive ink out of an ink firing chamber.
Thermal systems are limited in resolution (minimum drop size) by the lack of control over the bubble generation process, and in delivery rate by the recovery/cooling cycle and time for ink to refill the void left by the bubble. The ink compositions must also be such as to withstand the thermal cycling of the system.
Piezoelectric devices are relatively expensive to build and limited in firing rate, typically in the order of MHz, by the response times of the material.
In both cases it is difficult to further miniaturize the systems. In particular, for piezo-crystals it is difficult to make them smaller whilst increasing their operating frequency and generating sufficient deflection to drive ink ejection.
At the same time, it is known that laser generated acoustic waves in solids can be produced by two mechanisms. In the thermoelastic regime, which occurs at low laser power densities, laser induced temperature rises produce rapid thermal expansion and transient acoustic waves. Such waves were detected early in the development of laser processing, see xe2x80x98Calorimetric and acoustic study of ultraviolet laser ablation of polymersxe2x80x99, G. Gorodetsky et al, Appl. Phys. Lett. Vol 46 (1985) pp 828-830. Laser generation of ultrasound in the thermoelastic regime is a nondestructive process. However, in the ablation regime, which occurs at high peak powers, the recoil forces generated by vaporized material leaving the sample generate strong acoustic waves or shock waves. This regime involves the removal of very thin layers of material, although this layer may be a renewable material such as an oil or liquid coating.
EP1008451A2, xe2x80x98Laser-Initiated Ink-Jet Printing Method and Apparatusxe2x80x99, filed Dec. 11, 1999 describes a laser driven ink jet printing head relying on the laser generation of acoustic waves. In this system, single or possibly multiple scanning laser beams are each focussed to generate respective acoustic waves in a liquid contained in a first chamber located above an ink-firing chamber. The acoustic waves are transmitted from the first chamber to the ink-firing chamber through an intermediate body which lies between the chambers and which is almost transparent to the acoustic waves. When the transmitted acoustic wave enters the ink-firing chamber, it causes a droplet of ink to be ejected from a nozzle lying in register with the focussed laser beam. However, in this system, the intermediate body must have sufficient thickness and strength to protect the ink chamber from the pressure perturbations generated by bubble formation and collapse etc in the first chamber, as well as being able to act as an acoustic window allowing acoustic waves to be transmitted to the firing chamber. These two requirements (acoustic window and protective barrier) limit the type and thickness of material that can be used for the intermediate body. These are major disadvantages to the cost and flexibility of a commercial inkjet printing system.
According to the present invention there is provided an inkjet print head as claimed in claim 1.
The present invention relies on laser produced pressure waves to generate a driving pressure for ink expulsion. In this sense it could offer the advantages of piezoelectric devices, in that the mechanism is independent of ink chemistry. However, laser devices can be operated at variable pulse repetition rates of up to GHz, overcoming the firing rate limitations of piezoelectric devices. The laser pulse energy is also infinitely variable, offering fine control over the driving pressure and droplet size.
Preferably the wall comprises a membrane. The prior art makes no reference to the provision within a print head of a laser cooperating with a membrane or to the permanent physical displacement of ink with the resultant pressure waves.
The present invention overcomes many of the problems of conventional inkjet mechanisms. In relation to firing rate limitations, it will be seen that in principle the firing rate of the print head can be increased to the pulsing rate of the laser source, currently GHz and increasing in the future.
By using acoustic emissions to directly expel the ink, rather than heat as in the prior art, the requirement of the ink chemistry to withstand rapid temperature cycling is mitigated, allowing a broader range of ink chemistries to be employed within inkjet print heads.
Because the laser sources used to generate the driving force for ink ejection are in general highly controllable, the print head provides more control over droplet size and speed.
Preferably, the laser sources are based on semiconductor technology and therefore readily miniaturized and integrated into electrical systems.
In principle the size of the laser generated acoustic source is limited by the focussability of the laser (around the wavelength of the laser light). In implementing the present invention, a smaller focussed spot or output beam could be an advantage, as for a given laser pulse energy this increases the energy density and this could generate more driving pressure for ink ejection.
This miniaturization means higher nozzle densities should be possible, thus increasing print head resolution.
The present invention differs from EP1008451 in the following respects:
1. The invention generates acoustic waves, preferably using a thermoelastic mechanism, in a solid material rather than an opto-acoustic effect in a liquid.
2. The invention generates the acoustic wave directly in a membrane. The superficially similar intermediate body in EP1008451 is an acoustic window and plays no part in the generation of the acoustic wave.
3. The membrane of the invention need not be made of an acoustically transparent material, if sufficiently thin. The intermediate body in EP100851 must be acoustically transparent and cannot be made arbitrarily thin as it also acts to protect the ink chamber from pressure perturbations associated with bubble generation in the buffer solution.
4. Since the membrane used in the current invention is a solid material, bubble formation does not result after deposition of the laser energy. Therefore, there is no disruption to further laser pulses, which might limit the firing rate of the system.
5. The current invention does not require an optical system to focus and distribute the laser beam as required for EP1008451. Neither does it use a buffer liquid. The print head is much simpler with fewer moving parts.
6. In a preferred embodiment of the invention, the laser sources are integrated into the print head and therefore there is no requirement to synchronize the movement of the laser, laser beam and print head mechanism to allow full page width coverage, again making the print head of the invention much simpler.